Digital-to-analog converters (DACs) find application in myriad electronic applications. In some applications, it is important that the conversion from digital to analog be highly accurate. Exemplary are signal analysis instruments that must produce highly accurate analog excitation signals from corresponding digital signal data. In most such applications, conversion errors that are a function of the input digital data (hereafter referred to as distortion) are much more important than errors that are uncorrelated (hereafter referred to as noise). The noise portion of any conversion error can always be reduced by averaging the waveform over time. The distortion portion of the error, however, cannot.
To achieve low distortion D/A conversion, resort must usually be made to precision DACs that have been fabricated with finely matched components. An alternative approach is to quantify the distortion error of a particular converter at all possible input signal conditions and then to implement a correction circuit that compensates for the circuit's known error. Both approaches, however, are expensive and unsuitable for large volume production.
The present invention achieves accuracy comparable to that achieved with the precision approaches, but employs readily available off-the-shelf components.
According to the present invention, the input signal to be applied to a conventional DAC is first processed so that its value is uncorrelated with that of the unprocessed input signal. This processing can be effected by adding a digital random, or pseudorandom number to the input digital signal. The digital random number is then converted into analog form by a first DAC, and the processed sum is converted into analog form by a second DAC. An analog subtraction circuit then subtracts the analog version of the random number from the analog version of the processed sum. The difference is the analog counterpart to the original digital input signal.
For any digital input sample, the output of the first DAC is equally likely to contain any of the possible distortion errors produced by that DAC. The same is true for the output of all except the most significant bit of the second DAC. The most significant bit has some correlation to the input signal. To segregate the effects of this bit, it is desirably stripped off and applied to a third, one-bit DAC. This leaves the outputs of the first and second DACs both uncorrelated with the original digital input signal, meaning their output error signals are random sequences (i.e. noise) composed of DAC distortion errors. The ensemble of this noise signal is a small constant, termed an offset error. If desired, this small offset error can be removed by subsequent processing stages.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description thereof, which proceeds with reference to the accompanying Figure.